(a) Field of the Invention
The present invention relates to a hydraulic control system for automatic transmissions used in vehicles.
(b) Description of the Related Art
Conventional automatic transmissions used in vehicles include a torque converter, a multi-stage gear shift mechanism connected to the torque converter, and a plurality of friction members actuated by hydraulic pressure for selecting one of the gear stages of the gear shift mechanism, the gear shift mechanism being realized through a planetary gearset. The friction elements are controlled to engaged and disengaged states by a hydraulic control system, which controls pressure generated in an oil pump, to change shift ratios of the planetary gearset.
The friction elements are selectively operated by a plurality of valves, which undergo port conversion to change the flow of hydraulic pressure, and actuators supplying hydraulic pressure to the valves. Further, a manual valve, indexed with a driver-operated shift selector to realize port conversion, is connected to a plurality of lines to supply hydraulic pressure from the oil pump to each valve and actuator.
Korean Patent No. 97-25542 (Jun. 18, 1997), filed the present applicant, discloses such a hydraulic control system. However, in this application, to realize four forward shift ranges and one reverse shift range, four large-capacity solenoid valves, and a pressure control valve for controlling the solenoid valves, are used as a hydraulic pressure distributor. However, these solenoid valves are excessively large, and a valve body of the solenoid valves has an internal line structure that is complicated. Further, since a line pressure is controlled only by a drive pressure when driving in forward speeds, and a reverse pressure when driving in reverse, there occurs drive loss of the hydraulic pump during high-speed driving. This also results in a higher level of fuel consumption.